selftests: forwarding: add wait_for_dev() helper

The existing setup_wait*() helper family check the status of the
interface to be up. Introduce wait_for_dev() to wait for the netdevice
to appear, for example after test script does manual device bind.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>